The present invention relates generally to a roller apparatus for generating a substantially uniform charge on a surface, and, more particularly, concerns a biased roll charging apparatus having a clipped AC input voltage with a DC offset voltage.
When used to charge an imaging member, a roller used to create a charge on a another surface or substrate is commonly referred to as bias charge roll (xe2x80x9cBCRxe2x80x9d). When used to charge a substrate to enable transfer of a developed image from an imaging member to a substrate member, a roller used to create such bias charging is commonly referred to as a bias transfer roll (xe2x80x9cBTRxe2x80x9d). Although both may differ in details particular to their applications, both represent illustrative embodiments of the present invention.
Generally, the process of electrostatographic reproduction is initiated by substantially uniformly charging a photoreceptive member, followed by exposing a light image of an original document thereon. Exposing the charged photoreceptive member to a light image discharges a photoconductive surface layer in areas corresponding to non-image areas in the original document, while maintaining the charge on image areas for creating an electrostatic latent image of the original document on the photoreceptive member. This latent image is subsequently developed into a visible image by a process in which a charged developing material is deposited onto the photoconductive surface layer, such that the developing material is attracted to the charged image areas on the photoreceptive member. Thereafter, the developing material is transferred from the photoreceptive member to a copy sheet or some other image support substrate to which the image may be permanently affixed for producing a reproduction of the original document. In a final step in the process, the photoconductive surface layer of the photoreceptive member is cleaned to remove any residual developing material therefrom, in preparation for successive imaging cycles.
The above described electrostatographic reproduction process is well known and is useful for both digital copying and printing as well as for light lens copying from an original. In many of these applications, the process described above operates to form a latent image on an imaging member by discharge of the charge in locations in which light from a lens, laser, or LED discharges a charge. Such printing processes typically develop toner on the discharged area, known as DAD, or xe2x80x9cwrite blackxe2x80x9d systems. Light lens generated image systems typically develop toner on the charged areas, known as CAD, or xe2x80x9cwrite whitexe2x80x9d systems. The embodiments of the present invention apply to both DAD and CAD systems.
With respect to BCR applications, those skilled in the art recognize that various devices and apparatus have been proposed for creating a uniform electrostatic charge or charge potential on a photoconductive surface prior to the formation of the latent image thereon. Generally, corona generating devices are utilized to apply a charge to the photoreceptive member. In a typical device, a suspended electrode, or so-called coronode, comprising a thin conductive wire is partially surrounded by a conductive shield with the device being situated in close proximity to the photoconductive surface. The coronode is electrically biased to a high voltage potential, causing ionization of surrounding air which results in the deposit of an electrical charge on an adjacent surface, namely the photoconductive surface of the photoreceptive member. Corona generating devices are well known, as described, for example, in U.S. Pat. No. 2,836,725, to R. G. Vyverberg, among numerous other patents and publications. In the referenced Vyverberg patent, the coronode is provided with a DC voltage, while the conductive shield is usually electrically grounded and the photoconductive surface to be charged is mounted on a grounded substrate, spaced from the coronode opposite the shield. Alternatively, the corona device may be biased in a manner taught in U.S. Pat. No. 2,879,395, wherein the flow of ions from the electrode to the photoconductive surface is regulated by an AC corona generating potential applied to the conductive wire electrode and a DC potential applied to the conductive shield partially surrounding the electrode. The DC potential allows the charge rate to be adjusted, making this biasing system ideal for self-regulating systems. Various other corona generating biasing arrangements are known in the art and will not be discussed in great detail herein.
Several problems have historically been associated with corona generating devices. One problem includes the use of very high voltages (3000-8000 V), requiring the use of special insulation, inordinate maintenance of corotron wires, low charging efficiency, the need for erase lamps and lamp shields and the like, arcing caused by non-uniformities between the coronode and the surface being charged, vibration and sagging of corona generating wires, contamination of corona wires, and, in general, inconsistent charging performance due to the effects of humidity and airborne chemical contaminants on the corona generating device. More importantly, corotron devices generate ozone, resulting in well-documented health and environmental hazards. Corona charging devices also generate oxides of nitrogen which eventually desorb from the corotron and oxidize various machine components, resulting in an adverse effect on the quality of the final output print produced thereby.
As an alternative to corona generating devices used in charging systems, roll charging systems such as, BCR""s and BTR""s have been developed and incorporated into various machine environments with limited success. BCR charging systems are exemplified by U.S. Pat. No. 2,912,586 (to R. W. Gundlach); U.S. Pat. No. 3,043,684 (to E. F. Mayer); U.S. Pat. No. 3,398,336 (to R. W. Martel et al.); U.S. Pat. No. 3,684,364 (to F. W. Schmidlin); and U.S. Pat No. 3,702,482 (to Dolcimascolo et al.), among others, wherein an electrically biased charging roller is placed in contact with the surface to be charged, e.g. the photoreceptive member. Also relevant is U.S. Pat. No. 5,412,455, to Ono et al. wherein a charging device includes: a member to be charged; a charging member connectable to the member to be charged; a power source for supplying an oscillating voltage to the charging member; and a constant voltage element connected electrically in parallel with the power source for generating the oscillating voltage. Also, U.S. Pat. No. 5,463,450, to Inoue et al. discloses a charging apparatus for electrically charging a member to be charged including a charging member contactable to the member to be charged. The member to be charged includes a core and a voltage source for applying an oscillating voltage between the member to be charged and the charging member, wherein the frequency of the oscillating voltage satisfies a predetermined condition. Each of these is hereby incorporated by reference in their entirety.
In BCR charging systems, a charging member in the form of a roller is contacted with the surface of the photoreceptive member or other member to be charged, and an oscillating input voltage, typically a DC biased AC voltage signal, is applied to the roller to generate an oscillating electric field for applying a charge potential of a given polarity, to the photoreceptive member where the DC offset defines the polarity of the charge applied. Although the input voltage may be comprised solely of a DC component, an oscillating voltage such as, an AC voltage signal having a DC voltage signal superimposed thereon has been found to be preferable with respect to charge uniformity. See, for example, U.S. Pat. No. 4,851,960 to Nakamura et al which teaches that peak-to-peak input voltage, Vp-p, for DC-biased AC wave form should be at least twice the charge starting voltage for the photoreceptor or other charge receptor in the system being charged.
The absence of charge uniformity tends to manifest itself in the form of periodic stripes or so-called strobing corresponding to the variation in charge potential on the photoconductive surface. This strobing effect causes variations in toner attraction during development and often results in significant image quality degradation. However, an oscillating input voltage contributes both positive and negative polarity charges to the photoconductive surface. This results in a charging system that requires relatively high charging and discharging currents which, in turn, has a negative effect on the functional life the photoreceptive member. Also, high oscillating charging voltage induces complementary corona charges. Experience indicates that positive corona charges coupled with oscillating discharge increase photoreceptor member wear. Thus, a significant disadvantage of most biased roll charging systems is the resulting rapid wear of the photoconductive surface caused by the electrical discharge from the bias charge roll during the charging process. A related cause for rapid wear appears to be chemical degradation of organic and other complex molecules coupled with repetitive wiping or scraping of the photoreceptor layers by cleaning blades or other cleaning members.
One partial solution to the above problems is found in U.S. Pat. No. 5,613,173, issued to Kunzmann et al., hereby incorporated by reference in its entirety. In Kunzmann, a BCR apparatus is disclosed having clipped AC input voltage to reduce the phenomenon of strobing while also reducing photoreceptor wear caused by the electrical discharge from the bias charge roll during the charging process. The clipping of the AC oscillating voltage removes one polarity from the input signal, thereby supplying a single polarity to the photoreceptor or other charged member and, as a result, enabling sufficient charging at lower voltages applied to the charged surface. Such lower voltages extend photoreceptor life, in part by reducing electrically induced chemical damage.
Testing and experience has shown that the clipped AC BCR invention of Kunzmann increases photoreceptor life by approximately 15-40% when compared to unclipped AC BCR systems of the same peak voltage, current, and oscillating frequency. Since photoreceptor life is one of the primary parameters establishing the useful life of a typical customer replaceable cartridge (CRU) containing a photoreceptor, further extensions of photoreceptor life directly extend CRU life cycles and, thereby, significantly affect overall cost of ownership of electrophotographic printing systems using BCR systems.
Although Kunzmann describes a BCR system that improves photoreceptor useful life by decreasing photoreceptor wear, it would be advantageous to create a BCR system that greatly improves photoreceptor useful life even more than the invention in Kunzmann.
In accordance with the present invention, one aspect of the invention is an apparatus for applying an electrical charge to a member to be charged, comprising: (a) a power supply for supplying an oscillating voltage signal; (b) a charge roll member situated in proximity to a surface of the member to be charged; (c) a device for removing a selected polarity component of the oscillating voltage signal, thereby rectifying the signal; and (d) a DC power source for superimposing a DC bias signal upon the rectified oscillating voltage signal, wherein a voltage signal comprised of a rectified waveform with a DC bias offset is supplied to the charge roll member.
In accordance with another aspect of the invention, an electrostatographic imaging system is provided, comprising: (a) a member to be charged; (b) a power supply for supplying an oscillating voltage signal; (c) a charge roll member situated in proximity to a surface of the member to be charged; (d) a device for removing a selected polarity component of the oscillating voltage signal, thereby rectifying the signal; and (e) a DC power source for superimposing a DC bias signal upon the rectified oscillating voltage signal, wherein a voltage signal comprised of a rectified waveform with a DC bias offset is supplied to the charge roll member.
In accordance with yet another aspect of the invention, a process for applying an electrical charge to a member to be charged is provided, comprising: providing an oscillating voltage signal; removing a selected polarity component of the oscillating voltage signal, thereby rectifying the signal; superimposing a DC bias signal upon the rectified oscillating voltage signal; and supplying to the charge roll member voltage signal comprised of a rectified waveform with a DC bias offset.